What are the advantages and disadvantages of using postal questionnaires as a research method?

Advantages of using a postal questionnaire include the fact that it is not as time consuming as an interview. Questionnaires can be designed to be quick and easy for the respondant to complete. It can also be useful as you can distribute it to a larger sample. By posting it you can send it to anywhere desired. 

However the response rate can be a disadvantage of postal questionnaires, effecting the reliability of it. As people recieve it in the post it may not be seen as important and therefore do not respond. Those who do respond may only show a partial representative of people who have the time to complete.

Alongside this, unlike an interview, if the respondant doesn't understand the question or what needs to be answered they cannot confer with you as they complete it in their home. This may harm the validity of their answers as they may not understand the question without prompts.